Method and material for coating metal surfaces



Patented July 26, 1932 PATENT OFFICE JOHN S. THOMPSONAND ROBERT R. ANNER, OF DETROIT, MICHIGAN, ASSIGNORS TO PARKER RUST-PROOF COMPANY, 10F DETROIT, MICHIGAN, A CORPORATION OF MICHIGAN METHOD AND MATERIAL FOR COATING METAL SURFACES No Drawing.

This invention relates to coating metal surfaces and more particularly to a method by which metal surfaces are coated with a substantially insoluble coating of metal phosphates, more particularly where some metal less basic than the surface coated is included in the coating.

The object of the invention is to afford a method and material whereby a coating of the character described can be quickly and reliably formed upon metal surfaces. Details of the invention and of its purposes will appear as the description proceeds.

It has been known hitherto that a coating desirable particularly as a foundation for paint or enamel may be formed upon metal surfaces by immersing them in a boiling solution containing acid phosphates of manganese and/or iron, together with a small amount of a soluble compound of a metal less basic than the metal to be coated. While other metals may be employed to some extent and under some circumstances for this less basic material, the preferred metal is copper. \Vith the solutions used, only a minute proportion of the total solution was of this less basic metal, the most satisfactory results being obtained with the amount of this metal being of the order of 3/1000ths of 1% of the total solution.

In the commercial application of the method with the above described solution, it was found that in starting a new tank, after the first day or so, trouble was encountered because of the soft loose character of the coating produced. This trouble continued for sometime, depending upon the amount of work run through the solution. After a time, varying from a few days to several weeks, the solution seemed to once more right itself so that a satisfactory coating was formed. The soft, loosely adherent coating will be referred to hereinafter as smudge.

When dihydrogen phosphates of manganese and/or iron are dissolved in water and boiled therein, there is formed some less- I acid phosphates and at the same time at least some phosphoric acid is freed. For that reason, in the working solution of phosphates there is always present some phos- Application filed February 21, 1931. Serial No. 517,633.

phoric acid. The proportion of free phosphoric acid to the acid in the phosphates may be ascertained by titration in the follow- 1ng manner:

A 10 cc. sample of the solution may be taken wlth both methyl orange and phenolphthalein indicator, then alkali, such as tenth normal sodium hydroxide, is added, the amount added before the methyl orange indicator is affected giving the amount of free acid in the solution, while the amount added to affect the phenolphthalein indicator gives the 3 total acid in the bath, including that in the phos phates. It has been found that the ratio of the total acid to the free acid, in the type of bath indicated above, should be as high as 6 to 1. If a lower proportion is found, that is, if the total acid in the solution is only 4 or 5 to 1 of the free acid, the solution otherwise being suitable for producing the coating mentioned above, smudge results. By adding alkali to neutralize suflicient of the free acid so as to increase the proportion of total acid to free acid to at least 6 to 1, the smudge disappears and a satisfactory, firm, adherent coating is produced.

In the type of bath described above, the amount of the copper constituent of compounds dissolved in the bath should be between 2/1000ths of 1% of the total bath and 4/1000ths of 1% of the bath. A reduction below the lower amount results in a very much slower reaction, while an increase above the upper amount named results in a loose, unsatisfactory coating. Where the copper content of the solution is kept between 2/1000ths of 1% and 4/1000ths of 1%, and the proportion of total acid to free acid is kept as great as 6 to 1, a satisfactory coating is formed.

Increase of the proportion of total acid to free acid up to 8 or 9 to 1 decreases the time necessary for reaction, where the articles to be coated are immersed in the boiling solution. A further increase in the total acid of the solution in proportion to the free acid, conveniently obtained by neutralizing part of the free acid, does not materially decrease the time of reaction until the proportion .is around 12 to 1. A further decrease of the free acid results in a slowing up of the reaction and the bath is no longer satisfactorily active when the proportion of total acid to free acid is greater than about 15 to 1. From the above statements, it will appear that the proportion of total acid to free acid should be maintained between 6 to 1 and 15 to 1, in accordance with conditions.

While the preferred method ofemploying the solution is to immerse articles to be coated in the boiling solution, there are occasions where this method of employingthe bath is inconvenient. It has been found that a successful coating can be produced by continuously spraying or flowing the solution onto the surface to be coated where the solution is kept hot and a proper proportion of free acid is maintained. If, in spraying, a solution containing a proportion of total acid to free acid of 6 or 7 to 1, which is found satisfactory for the immersion process, is employed, a very smudgy nonadherent coating is the result. By reducing the free acid so that the proportion of total acid to free acid is approximately 10 to 1, it has been found possible to obtain a very satisfactory coating by the flowing method in from two to three minutes, with no signs of smudge The free acid should not be reduced so as to make the proportion of total acid to free acid greater than about 12 to 1, for the flowing process, as

such decrease in the free acid slows up the reaction unnecessarily and without apparent benefit. 7

Where the solution is employed to coat wire or steel wool or other metal where the exposed surface is very great in proportion to the total amount of metal, using the regular coating bath containing a proportion of total acid of about 6 or 7 to 1 of the free acid, the coated metal is injuriously affected, being rendered somewhat brittle. It has been found that, by decreasing the free acid so that the proportion of total acid is around 12 to 1 of the free acid, or about twice as great as in the usual immersion solution, the coating of fine wire or steel wool may be efl'ected promptly without injuring the material and with the production of a satisfactory coatin To summarize, an acid phosphate bat containing from 2/1000ths of 1% to 4/1000ths of 1% of copper in compounds in solution, a

proportion of total acidity, as indi'cated'by phenolphthalein indicator, to the free acid, as indicated by methyl orange indicator, as great as 6,to 1 results in a firm adherent satisfactory coating; other conditions being the same, the proportion for a solution which is sprayed or flowed upon the article to be coated should be about 10 to 1. For'coating wire or steel wool or other articles likely to be rendered brittle by treatment with acid, the proportion of total acid to free acid is preferably about 12 to 1. In each case the solution may be tested in the manner described above and the proper proportion of total acid to free acid ma be obtained by partially neutralizing the ree acid until the desired proportion results.

While the method has been described as primarily applicable to the coating of iron phosphate bath where no metal less basic than that to be coated is employed.

In a rust-proofing bath formed and replenished by dissolving therein dihydrogen phos phate of iron, and/or manganese, the proportion of free acid is higher than it should be for most rapid coating formation. If any free acid is used in forming or replenishing the bath, this condition is aggravated. The speed of operation can be increased by partially neutralizing the free acid, the degree of neutralization depending to some extent upon the metal used in the phosphate, the nature of the surface being coated, etc. .It has been found that ordinarily the most effective ratio is between 1 of free acid to 7.5 of total acidity and 1 of free acid to 10 of total acidity.

Where the bath contains some metal less basic than the metal to be coated, it is necessary to avoid smudge in order to obtain a satisfactory coating. Where thebath is free from such less basic metal, a proportion of free acid as high as that normally present in a bath replenished by dihydrogen phosphate, does not raise difficulties because of the nonadherence of the coating, but partial neutralization may be employed where conditions render the resultant speeding up of the coating operation sufficiently important.

Various substances may be employed for the partial neutralization necessary to obtain the desired ratio between free acid and total acidity, but care must be taken to avoid the use ofv a material which is ultimately detrimental to the bath, if the addition of the material is to be continued regularly. There is y a much wider choice of materials when the It will be understood, therefore, that while this invention relates more particularly to a solution containing some metal less basic thanthat to be coated, its broader aspect applies also to the correct proportion of. the free acid to the total acid to obtain the most effective phosphate solution for producing a phosphate. coating rapidly, or where the metal is likely to be allectcd injuriously by acid and approximately semineutralization is employed.

What we'claim is:

1. The method of coating metal of the group comprising magnesium, zinc, iron, and their alloys with a phosphate solution, which comprises testing the solution for free acid and for total acidity, neutralizing the free acid until a proportion of total acid to free acid of between 6 to 1 and 15 to 1 is obtained, and subjecting a metal surface to the solution having a proportion of total acid to free acid within said range.

2. A method in accordance with claim 1 and in which the proportion of total acid to free acid is between 7 to 1 and 10 to 1.-

3. A method of coating metal of the group comprising magnesium, zinc, iron, and their alloys with a phosphate solution containing in solution a small amount of metal less basic than the metal to be coated, which comprises testing the solution for total acid and for free acid, neutralizing free acid if necessary to obtain a proportion of total acid to free acid of between 6 to 1 and 15 to 1, and treating a metal surface with the solution having a proportion of total acid to free acid within said range.

4. The method of coating metal of the group comprising magnesium, zinc, iron, and their alloys with a phosphate solution containing in solution a small amount of metal less basic than the metal to be coated, which comprises forming a solution of-the character described, determining the proportion of total acid in the solution to free acid therein, neutralizing the free acid until the proportion of total acid to free acid is approximately 10 to 1, and flowing the solution upon the surface to be coated While maintaining the sol ution and surface at the point of contact at a raised temperature.

5. The method of coating metal of the group comprising magnesium, zinc, iron and their alloys with a solution of dihydrogen phosphate of iron, and/or manganese, which comprises forming a solution of the character described, ascertaining the proportion of total acid in the solution to free acid resulting from the normal hydrolization in the solution, neutralizing the free acid until its proportion to the total acid is about one half as great as resulted from said hydrolization,

and treating therewith metal likely to be affected injuriously by treatment with free acid.

6. The method of coating'metal of the group comprising magnesium, zinc, iron and their alloys with a phosphate solution containing in solution a small amount of metal neutralizing the free acid until the proportion of total acid to free acid is approximately twice as great as results from normal changes within the solution, and treating therewith metal likely to be injuriously affected bythe treatment with acid.

7. The method of coating metal surfaces, which consists in forming a solution. of acid phosphates of manganese and/or iron and containing copper as a constituent of a composition in solution to the amount of between 2/1000ths of 1% and l/lOOOths of 1% of the total solution, which comprises testingv the solution for total acid and for free acid,

neutralizing free acid until the desired proportion of total acid to free acid is obtained, and treating withthe solution a surface of metal of the group comprising iron, zinc and magnesium or their alloys.

8. The method of coating metal surfaces, which consists in forming a solution of acid phosphates of manganese and/or iron and containing copper as a constituent of a composition in solution to the amount of between 2/1000ths of 1% and 4/1000th's of 1% of the total solution, which comprises testing the solution for total acid and for free acid, neutralizing free acid until a proportion of total acid to free acid of between 6 to 1 and 15 to 1 is obtained. and treating with the solution a surface of metal of the group comprising iron, zinc and magnesium or their alloys.

9. The method of coating metal of the group comprising magnesium, zinc, iron, and their alloys, surfaces, which consists in forming a solution of acid phosphates of magnesium and/or iron and containing copper as a constituent of a composition in solution to the amount of between 2/1000ths of 1% and 4/1000ths of 1% of the total solution, which comprises forming a solution of the character described, determining the proportion of total acid in the solution to free acid therein, neutralizing the free acid until the proportion of total acid to free acid is approximately 10 to 1, and flowing the solution upon the surface to be coated while maintaining the solution and surface at the point of contact at a raised temperature.

10. The method of coating metal of the group comprising magnesium, zinc, iron, and their alloys, surfaces, which consists in forming a solution of acid" phosphates of manganese, and/or iron and containing copper as a constituent of a composition in solution to the amount of between 2/1000ths of 1% and 4/1000ths of 1% of the total solution,

Which comprises forming a solution of the character described, ascertaining the proporv tion of total acid in the solution to free acid therein, neutralizing the free acid until the proportion of total acid to free acid is approximately twice as great as results from normal changes within the solution, and treating therewith metal likely to be injuriously afi'ected by the treatment with acid.

11. A solution for coating surfaces of iron, steel or zinc comprising a dilute solution of phosphates of metal as basic as iron and not more than one-tenth as much phosphate of copper in the solution, and a neutralizing reagent in quantity to neutralize substantially one-half of the acidity which would other- I 1 wise be present in the heated bath.

12. The method of coating surfaces of iron, v

steel or zinc which comprises forming a dilute solution of phosphates of a metal or metals at 'least as basic asiron and a phosphate of copper, the amount of copper in the solution a being not more than one-tenth of the metal as basic as iron in the solution, and neutralizing the bath to about-one-half of the acidity which it would otherwise have, heating the 7 solution and subjecting the surfaces tobe coated to the action of said solution.

In testimony whereof we have hereunto signed our names to this specification.

- JOHN S. THOMPSON. ROBERT R. TANNER- 

